A few blood stains and bullet holes never stopped you from making a proper disguise. When critters with outfits die near you, you will often be able to salvage a piece of their outfit for use with a disguise kit.

E.g.: Ted has his own Blackspear Tribal disguise, but he wants to take his CNPC pal, Destructo, into the bunker as well. A random encounter finds him watching a group of Blackspears fighting raiders. Ted and Destructo sit the combat out until the Blackspears get annihilated. Ted then walks up and gingerly searches the bodies, finding a Blackspear headdress, gooey black facepaint made of... something, and one of their patchwork brahmin-skin outfits.